A healthy spinal disc (intervertebral disc) is a fibroelastic structure with a non-compressible viscous center that articulates adjacent vertebrae. Due to its deformable geometry, the disc not only supports normal functional loads of the human body, but also evenly distributes the stresses applied during body movement and positioning. The disc interfaces with associated superior and inferior vertebrae via large surface areas known as vertebral endplates. Normally, vertebral endplates are thin regions of dense bone (e.g. 1 mm-3 mm) that support high stresses at articulating junctions.
Intervertebral discs and adjacent articulations progressively deteriorate with age. This natural degenerative process results in various degrees of pathological changes, mostly affecting the geometry and elasticity of a vertebral disc. In severe cases, reduced disc volume results in foraminal compression that mechanically irritates nerve roots and causes neurocompressive syndrome. This often causes severe chronic pain that can only be resolved surgically.
Historically, surgical treatment of degenerative spinal disc disease required fusion, which immobilizes two adjacent vertebral bodies (vertebrae) to prevent motion-sensitive pain and inflammation. This is accomplished by distracting the vertebrae to a healthy disc height, inserting a disc implant and allowing bone to grow between and through the disc implant until the vertebrae fuse into a solid bony structure. To facilitate proper healing under normal conditions of motion, the disc implant is used to maintain temporary positioning until the bone achieves fusion. The implant is secured to the vertebrae using fixation elements.
The effectiveness of the disc implant can be evaluated with the following criteria: (i) its ability to restore and maintain normal disc height and curvature; (ii) its ease of delivery and fixation to the disc space; (iii) its ability to facilitate fusion of associated vertebrae; and (iv) its ability to restrict movement of associated vertebrae.
Disc implants share the same fundamental characteristics to meet the effectiveness criteria. Implants aim to restore disc height through the use of variable geometries. Lordotic curvature is preserved through the use ergonomic designs that conform to spinal curvature and height between the vertebrae. Also, the disc implants are sufficiently porous or hollow to promote the growth of vertebral bone into and through the implant. However, independently, these implants can only restrict spinal flexion and intervertebral compression. Any excessive lateral, sliding, or extension motion may cause device failure and/or extrusion. To avoid this risk, it is customary to provide additional fixation of the disc implant to the vertebrae.
Devices and systems may integrate fixating members directly into the disc implant. These implants have garnered the nickname “standalone” due to their ability to self-fixate without the use of secondary fixation elements. In the foregoing standalone implants, obtrusive fixation elements are delivered directly through implant pilot openings into the vertebra, which fixate the implant to the vertebrae and prevent implant failure under remaining ranges of motion (e.g., lateral, sliding, extension). Nevertheless, during these motions, connectivity between fixation elements and vertebrae may become weakened causing the fixation elements to slip or extrude out of the implant. To prevent unwanted fixation element slipping or extrusion, it is customary to include a locking mechanism for the implant.